The ionization energy is:

Explanation

Ionization energy is the energy required to remove an electron from an atom in the gaseous state. As you move from left to right across a period, the number of protons in the nucleus increases, creating a greater attractive force on the electrons. This results in an increase in ionization energy. Conversely, as you move down a group, the outer electrons are further from the nucleus and experience more shielding from inner electron shells, which decreases the ionization energy. Therefore, the correct answer is that ionization energy increases from left to right in a period. The other options are incorrect because they either suggest ionization energy remains constant or incorrectly describe its trend across a group.